The long term goal of this research is to design exercise interventions to reduce major adverse cardiac events (cardiac morbidity and mortality) and costs of care in individuals who have ventricular arrhythmias with an implantable cardioverter defibrillator (ICD). The goal of this study is to determine if an aerobic exercise intervention in addition to usual care, will significantly improve cardiopulmonary function, cardiac autonomic activity, and quality of life while reducing ventricular arrhythmias, inflammation, and psychological distress in patients with ventricular arrhythmias who have an ICD for the secondary prevention of sudden cardiac arrest (SCA). Aerobic exercise is known to increase parasympathetic cardiovascular regulation and reduce cardiac mortality. In our pilot study, we demonstrated that aerobic conditioning, strenuous enough to improve cardiopulmonary function, could be safely implemented in this population. The specific aims of this study are to test the hypotheses that: 1) aerobic exercise (5 hours/week for 8 weeks home walking at 60- 80% of peak V02 HR followed by 16 weeks of exercise maintenance) compared to usual care alone will increase cardiopulmonary function (peak V02), 2) aerobic exercise will increase cardiac autonomic activity (heart rate variability (HRV) and health related quality of life (SF-36 and Patient Concerns), while reducing ventricular arrhythmias (ICD shocks, VF/VT), psychological distress (anxiety and depression), and inflammation (hsCRP, BNP, TNF-alpha, IL-6);and 3) aerobic exercise will mediate effects on outcomes through improvement in cardiopulmonary function. Using a randomized clinical trial format, an aerobic exercise program in addition to usual care will be tested against usual care alone. The exercise intervention is designed to be conducted at home after a cardiopulmonary exercise test, using careful telephone and Polar Heart rate monitoring. The primary analysis for treatment effect is improvement in cardiopulmonary function at the end of 8 weeks. Longitudinal effects of the program will be assessed at 6 months. Secondarily, describing the effects of aerobic exercise on other outcomes will be determined. Study findings are expected to be useful clinically in making exercise prescriptions after an ICD and to be generalizable to populations at risk for recurrent SCA who have an ICD.